Microstructure and hot corrosion behavior of Al-Si-Hf coating on new γ′-strengthened cobalt-based superalloy

An Al-Si-Hf coating was deposited on the new γ′-strengthened cobalt-based superalloy using pack cementation method. The coating was composed of an outer layer consisting of AlCo phase dispersed with Al₅Co₂ phase and a diffusion zone. The hot corrosion behavior of the Al-Si-Hf coating exposed to 75 wt% Na₂SO₄ and 25 wt% NaCl at 900 °C for 100 h was studied. The total mass gain was only 0.29 mg/cm². It is suggested that the addition of Si facilitated the formation of the Al₂O₃ scale and contributed stable SiO₂ against the molten salts. Hf modification reduced the growth rate of the alumina scale by restraining Al ions outward diffusion, and improved the adhesion of the oxide scale by providing pinning effect.